[Tripy] Separate converting shape args from converting tensor args

[slyubomirsky]

Addresses issue #214 and separates shape argument conversion from the convert_inputs_to_tensors decorator. Since most uses of the shape argument conversion would only convert one argument (usually named shape), this decorator takes a list of arguments to convert rather than converting all arguments by default. It also does not accept arguments that aren't sequences of Python numbers or Tripy Tensors, since it would make less sense to allow external data formats to silently pass through in cases that aren't operator overloads.